Long Term Evolution (LTE) is an improved universal mobile telecommunication system (UMTS) that provides higher data rate, lower latency and improved system capacity. To provide high data rate in a frequency selective fading environment, the downlink transmission utilizes Orthogonal Frequency Division Multiple Access (OFDMA) at the physical layer. However, one of the drawbacks of OFDMA is its vulnerability to Carrier Frequency Offset (CFO). Typical frequencies drift of 10 ppm (10×10−6) of the local oscillator results in an offset of 25 kHz. LTE system employs a fixed subcarrier spacing of 15 kHz. As a result, The typical frequency drifting of 10 ppm corresponds to 1.67 subcarrier spacing, while a large frequency offset up to 25 ppm corresponds to over four-subcarrier spacing. Such large frequency offset presents challenges for cell search in the LTE system.
In the LTE system, an evolved universal terrestrial radio access network includes a plurality of base stations, referred as evolved Node-Bs (eNBs), communicating with a plurality of mobile stations, referred as user equipment (UE). A UE may communication with a base station or an eNB via the downlink and uplink. Cell search as well as synchronization in the LTE system is performed in each UE by using both the Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS). The LTE wireless cellular system is designed with orthogonal frequency domain multiple access (OFDMA) in the physical layer. The incoming user data bits are multiplexed onto the assigned sub-carriers in frequency domain and transmitted as a single time-domain signal in downlink. This is accomplished by an inverse fast Fourier transform (IFFT) on the user data bits. For facilitating cell search procedures, known bit patterns are transmitted in specific time and frequency slots (also known as resource elements) for the mobile devices to be able to identify the cell's timing and its associated identifier (cell ID). A mobile device after being powered on, attempts to measure the received wideband power for specific frequencies over a set of frequency bands. After ranking the frequencies based on received signal strength indicator (RSSI), UE attempts cell search using the downlink synchronization channels.
The cell search procedure in LTE system can be performed in three steps. The first step is carried out by correlating the received Primary Synchronization Signal (PSS) samples to determine the cell's group identity out of three possible values and its timing information by determining the 5 ms boundary of cell's signal transmission. The latter is due to the fact that PSS signal is transmitted as the last symbol in 0th and 10th slots of a 10 ms frame. The second step is correlating the received samples of the secondary synchronization signal and reference signal to determine the cell identifier and frame timing. The third step is to verify the cell identification.
The initial cell search is a key process for the UE to establish communication channels with the eNB. The overall synchronization performance is dominated heavily by a robust PSS detection. However, the tradition PSS design is designed to work at low frequency offset. To support large frequency offset up to 25 ppm, a small coarse frequency bin with bandwidth smaller than 3.75 kHz is necessary. Each frequency bin and a corresponding PSC forms a hypothesis or a candidate. A multi-try based adaptive peak selection procedure should be used to reduce the number of candidates. The number of “try”, for the peak selection procedure is adaptively adjusted based on a channel condition. The threshold for selecting the candidates is also adaptively adjusted. Further, when the Signal to Noise Ratio (SNR) is low, higher number of non-coherent accumulation is required. Therefore, the number of non-coherent accumulation should also be adaptively adjusted based on SNR. Other factors may be considered such as the complexity and system limitation in implementing the process. For example, the multiple peaks may be applied at a later stage such as in the fractional frequency estimation module, if the coarse frequency bin is not small enough due to complexity considerations.